A few general comments regarding the nature and intended use of vehicles such as are here described may be conducive to a better understanding of the invention. In the first place, the power available continuously from even a very athletic man is quite limited, about three-tenths horsepower being an acceptable figure. A man in good condition can row a boat for a considerable distance, but his sustained rate of movement is very low, perhaps three miles per hour for a flat-bottomed boat or five miles per hour for a canoe. Pedal-boats and boats propelled manually are both known.
The power required to propel a conventional boat is expended almost entirely in overcoming the fluid friction or drag between the hull of the vessel and the water, the latter of course supporting the weight of the vessel and its occupant by the principle of buoyancy. It is thus clear that the rate of motion of a hulled water vehicle, for any available driving power, is limited by drag. The use of both air screws and water screws to drive water-borne vehicles is well known.
In the field of power boats the use of hydrofoils has been developed to increase the speed obtainable from an engine of given power. Such vessels when at rest are supported by a buoyant hull of minimum size; when in operation at rated speed the action of the water on suitable hydrofoils raises the vehicle so that the hull no longer engages the water, thus reducing the drag exerted against the vehicle by the liquid medium to a minimum due to the hydrofoils, and hence increasing the speed at which the vehicle can be driven by an engine of known power.
A present-day sport of increasing popularity is the construction and operation of man-powered aircraft, that is, heavier-than-air vehicles propelled and supported solely by the exertions of the occupant. The occupant must cause the vehicle to fly at such a speed that the lift of its airfoils maintains it at altitude. Flights of very respectable distance have been accomplished in properly designed vehicles of this sort, including flight across the English Channel.